The invention relates generally to audio signal distribution systems, and more particularly relates to maintaining quality of audio reproduction in conditions of poor transmission quality.
In audio distribution systems where audio signals are transmitted digitally, the audio signal decoded from the transmission can quickly become unintelligible when poor transmission quality results in a large number of bit errors. However, there is a region between perfect reception without transmission errors and unintelligibility where the decoded audio signal represents a degraded level of audio quality. Nonetheless, when the decoded audio signal is output directly, the transmission errors are disruptive and can have a jarring effect on the listener.
In accordance with the present invention, the quality of a received audio signal is allowed to degrade gracefully with decreasing transmission quality by controlling the volume at which the received audio signal is output. Disruptive noise in the audio output caused by the transmission errors are thus made less perceptible. Preferably, the audio signal is increasingly attenuated in relation to the rate of transmission errors. As compared to muting the audio output at a subjectively determined bit error rate, this dynamic volume adjustment eliminates rapid fluctuations between full and muted volumes which also is disruptive and subjectively undesirable to listeners.
In one aspect of the invention, the transmission quality is assessed by detecting errors occurring in known bit patterns embedded in the audio signal. Since the state of the audio signal per se is not known a priori at the receiver, errors in the known bit patterns embedded with the audio signal are indicative of the transmission quality. In an embodiment of the invention illustrated herein for example, the known bit pattern is a synch bit embedded one synch bit per 16-bit word of an audio signal transmitted on a digital serial transmission link. The rate of errors is tracked by maintaining a count which is modified between upper and lower limits according to whether each occurrence of the known bit pattern is received with or without error, such as by incrementing for each occurrence without error and decrementing for each occurrence with error. The increment value, decrement value, upper limit and lower limit are parameters that can be selected subjectively for a particular application.
In another aspect of the invention, the count representing the rate of transmission errors is smoothed, such as by use of a filter or calculating a moving average of the count. The volume of the audio output is then varied according to the smoothed count value. This avoids varying the audio output volume at rates faster than the listener""s ability to perceive volume change.
Additional features and advantages of the invention will be made apparent from the following detailed description of a preferred embodiment which proceeds with reference to the accompanying drawings.